Radiation-sensitive silver halide emulsions employed in radiographic elements are comprised of a dispersing medium and silver halide microcrystals, commonly referred to as grains. As the grains are precipitated from an aqueous medium, a hydrophilic colloid peptizer is adsorbed to the grain surfaces to prevent the grains from agglomerating. Subsequently binder is added to the emulsion and, after coating, the emulsion is dried. The peptizer and binder are collectively referred to as the vehicle of an emulsion.
Gelatin and gelatin derivatives form both the peptizer and the major portion of the remainder of the vehicle in the overwhelming majority of silver halide radiographic elements. An appreciation of gelatin is provided by this description contained in Mees The Theory of the Photographic Process, Revised Ed., Macmillan, 1951, pp. 48 and 49:
Gelatin is pre-eminently a substance with a history; its properties and its future behavior are intimately connected with its past. Gelatin is closely akin to glue. At the dawn of the Christian era, Pliny wrote, "Glue is cooked from the hides of bulls." It is described equally shortly by a present-day writer as "the dried down soup or consomme of certain animal refuse." The process of glue making is age-old and consists essentially in boiling down hide clippings or bones of cattle and pigs. The filtered soup is allowed to cool and set to a jelly which, when cut and dried on nets, yields sheets of glue or gelatin, according to the selection of stock and the process of manufacture. In the preparation of glue, extraction is continued until the ultimate yield is obtained from the material; in the case of gelatin, however, the extraction is halted earlier and is carried out at lower temperatures, so that certain strongly adhesive but nonjelling constituents of glue are not present in gelatin. Glue is thus distinguished by its adhesive properties; gelatin by its cohesive properties, which favor the formation of strong jellies. PA1 Photographic gelatin is generally made from selected clippings of calf hide and ears as well as cheek pieces and pates. Pigskin is used for the preparation of some gelatin, and larger quantities are made from bone. The actual substance in the skin furnishing the gelatin is collagen. It forms about 35 per cent of the coria of fresh cattle hide. The corresponding tissue obtained from bone is termed ossein. The raw materials are selected not only for good structural quality but for freedom from bacterial decomposition. In preparation for the extraction, the dirt with loose flesh and blood is eliminated in a preliminary wash. The hair, fat, and much of the albuminous materials are removed by soaking the stock in limewater containing suspended lime. The free lime continues to rejuvenate the solution and keeps the bath at suitable alkalinity. This operation is followed by deliming with dilute acid, washing, and cooking to extract the gelatin. Several "cooks" are made at increasing temperatures, and usually the products of the last extractions are not employed for photographic gelatin. The crude gelatin solution is filtered, concentrated if necessary, cooled until it sets, cut up, and dried in slices. The residue, after extraction of the gelatin, consists chiefly of elastin and reticulin with some keratin and albumin. PA1 Gelatin may also be made by an acid treatment of the stock without the use of lime. The stock is treated with dilute acid (pH 4.0) for one to two months and then washed thoroughly, and the gelatin is extracted. This gelatin differs in properties from gelatin made by treatment with lime. PA1 Although collagen generally is the preponderant protein constituent in its tissue of origin, it is always associated with various "ground substances" such as noncollagen protein, mucopolysaccharides, polynucleic acid, and lipids. Their more or less complete removal is desirable in the preparation of photographic gelatin. PA1 (1) Photographic silver halide emulsion layers and other layers on photographic elements can contain various colloids alone or in combination as vehicles. Suitable hydrophilic materials include both naturally occurring substances such as proteins, protein derivatives, cellulose derivatives--e.g., cellulose esters, gelatin--e.g., alkali-treated gelatin (pigskin gelatin), gelatin derivatives--e.g., acetylated gelatin, phthalated gelatin and the like, polysaccharides such as dextran, gum arabic, zein, casein, pectin, collagen derivatives, collodion, agar-agar, arrowroot, albumin and the like . . .
In addition to the collagen and ossein sought to be extracted in the preparation of gelatin there are, of course, other materials entrained. For example, James The Theory of the Photographic Process, 4th Ed., Macmillan, 1977, p. 51, states:
Superimposed on the complexity of composition is the variability of composition, attributable to the varied diets of the animals providing the starting materials. The most notorious example of this was provided by the forced suspension of manufacturing by the Eastman Dry Plate Company in 1882, ultimately attributed to a reduction in the sulfur content in a purchased batch of gelatin.
Considering the time, effort, complexity and expense involved in gelatin preparation, it is not surprising that research efforts have in the past been mounted to replace the gelatin used in photographic emulsions and other film layers. However, by 1970 any real expectation of finding a generally acceptable replacement for gelatin had been abandoned. A number of alternative materials have been identified as having peptizer utility, but none have found more than limited acceptance. Of these, cellulose derivatives are by far the most commonly named, although their use has been restricted by the insolubility of cellulosic materials and the extensive modifications required to provide peptizing utility.
Research Disclosure, Vol. 365, September 1994, Item 36544, II. Vehicles, vehicle extenders, vehicle-like addenda and vehicle related addenda, A. Gelatin and hydrophilic colloid peptizers, paragraph (1) states:
This description is identical to that contained in Research Disclosure, Vol. 176, December 1978, Item 17643, IX. Vehicles and vehicle extenders, paragraph A. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England.
During the 1980's a marked advance took place in radiographic elements containing silver halide emulsions. Abbott et al U.S. Pat. No. 4,425,425 disclosed that dual coated radiographic elements employing spectrally sensitized high (&gt;8) aspect ratio tabular grain emulsions exhibited reduced crossover. Abbott et al U.S. Pat. No. 4,425,426 extended these observations to radiographic elements employing thin (&lt;0.2 .mu.m) intermediate (5-8) aspect ratio tabular grain emulsions. Dickerson et al U.S. Pat. No. 4,414,304 discloses radiographic elements containing tabular grain emulsions that are fully forehardened, resulting from the observation that tabular grain emulsions, unlike the emulsions previously employed in radiographic elements, exhibit high covering power characteristics that are minimally affected by increased hardening of the emulsion vehicle. Based on these advantages of dual coated radiographic elements an industry conversion to tabular grain emulsions has occurred.
In the descriptions of high bromide {111} tabular grain emulsions provided by Abbott et al and Dickerson, cited above, the vehicle disclosure of Research Disclosure Item 17643 was incorporated verbatim. Only gelatin peptizers were actually demonstrated in the Examples.
Despite the assumption that conventional vehicle selections are fully applicable to tabular grain emulsions, there have been some indications that some peptizer selections are particularly advantageous for tabular grain emulsions. Maskasky U.S. Pat. No. 4,400,463 disclosed the use of synthetic peptizers in combination with adenine to produce high (&gt;50 mole %) chloride tabular emulsions. Later Maskasky U.S. Pat. Nos. 4,713,320 and 4,713,323 demonstrated that high bromide and high chloride tabular grain emulsions could be improved by treating gelatin with an oxidizing agent.
Maskasky U.S. Pat. No. 5,284,744 taught the use of potato starch as a peptizer for the preparation of cubic grain silver halide emulsions, noting that potato starch has a lower absorption, compared to gelatin, in the wavelength region of from 200 to 400 nm. Maskasky '744 does not disclose tabular grain emulsions.
Problems to be Solved
Notwithstanding the industry acknowledged advances that tabular grain emulsions have made possible in dual coated radiographic elements, there have remained problems, some unique to dual coated radiographic elements and some that are shared with all elements that employ silver halide emulsions. Some of these problems have been assiduously pursued in the hope that a solution can be found while other problems have received little recent attention, being accepted as inescapable limitations of silver halide emulsions.
Silver halide emulsions intended for photographic applications have found it advantageous to reduce the thickness of the tabular grains to the lowest levels that can be reliably obtained. Therefore, recent interest has been directed to ultrathin (&lt;0.07 .mu.m in thickness) tabular grain emulsions. In radiography a minimum average tabular grain thickness of about 0.1 .mu.m is common. The greater thickness of the tabular grains in radiography is dictated by the warmer image tones produced as tabular grain thicknesses are reduced. In medical diagnostic imaging, the primary use of dual coated radiographic elements, radiologists prefer radiographic images with "cold" image tones. Warm image tone has represented an obstacle to realizing the advantages of tabular grain emulsions exhibiting average grain thicknesses of less than 0.1 .mu.m. The incorporation of ultrathin tabular grains into a dual coated radiographic element increases the surface to volume ratio of the tabular grains, allowing higher levels of spectral sensitizing dye per unit of silver to be employed, thereby reducing crossover.
The cost and inconvenience of the preparation of gelatin and gelatin derived vehicles has become generally accepted, since no more attractive substitutes have been found, despite investigations that have been spread over a century.
Conventional peptizers obtained from gelatin, cellulose and starch, when employed as aqueous peptizers in forming silver halide emulsions, exhibit levels of viscosity well above the viscosity of water. Further, viscosities increase markedly as temperatures are lowered to approach room temperature (nominally 20.degree. C.), and for this reason silver halide emulsion precipitations are typically undertaken in the temperature range of from 30.degree. to 90.degree. C.
The elevated viscosity levels imparted by these peptizers, even at the elevated temperatures employed for silver halide precipitation, interfere with reactant mixing to obtain uniform grain characteristics. For example, elevated viscosities work against uniform mixing on a microscale (micromixing) which is essential for uniform grain nucleation and growth. Nonuniformity in grain nucleation and, to a lesser extent, growth result in grain polydispersity, including the coprecipitation of grains that differ in their shape and size and, where multiple halides are being coprecipitated, their internal distribution of halides.
On a macroscale the elevated levels of viscosity create difficulties in scaling up the silver halide precipitations to convenient volumes for manufacturing purposes. Elevated levels of viscosity work against being able to sustain desired levels of bulk mixing of reactants as the total volume of the reaction vessel is increased.
The peptizer polymers, being of natural origin, contain mixtures of differing molecules, differing in weight and structure, not all of which are well suited to emulsion preparation. Further, the peptizers exhibit variations based on origin of the starting materials and can vary in composition over time, even when obtained from a single commercial source. Unwanted effects can be seen both in physical properties, such as turbidity, and in sensitometric properties, such as fog.
It is generally accepted that heating of silver halide emulsions is required to achieve chemical sensitization by any one or combination of middle chalcogen (i.e., sulfur, selenium and/or tellurium), noble metal (e.g., gold) or reduction sensitization. For achieve anywhere near maximum acceptable photographic speeds heating to at least about 50.degree. C. is typical, with maximum temperatures being limited only by ambient vapor pressures (e.g., boiling away of the aqueous component). At these elevated temperatures grain ripening is accelerated. This can lead to varied unwanted effects, depending upon the nature of the grains present in the emulsion and their intended end use. Ripening, for example, rounds grain edges and corners of surviving grains, eliminates smaller grains entirely, and can destroy useful grain characteristics (e.g., deleterious thickening of tabular grains can be produced by ripening). Particularly sensitive to unwanted ripening are ultrathin (thickness &lt;0.07 .mu.m) tabular grain emulsions, which can exhibit mean grain thickness increases of in excess of 30 percent (and much higher) when ripening occurs at conventional chemical sensitization temperatures. Further, elevated temperatures during grain precipitation can also accelerate unwanted ripening and degrade desired grain characteristics.
Finally, the starches that have been heretofore investigated as peptizers have been generally observed to be clearly inferior in their peptizing action. Additionally, conventional starch peptizers, as demonstrated by Maskasky U.S. Pat. No. 5,274,644, cited above, favor the formation of grains having {100} crystal faces, whereas high bromide tabular grains require {111} faces in the forms that have found acceptance in practical use.